[1][2] Skin has mesodermal cells which produce pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight.

One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly.

[3] In terms of surface area, the skin is the second largest organ in the human body (the inside of the small intestine is 15 to 20 times larger).

[4] One average square inch (6.5 cm2) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1,000 nerve endings.

It forms the waterproof, protective wrap over the body's surface, which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina.

The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, and basale.

The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain.

The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin's surface.

[11][12][13] Tissue mass is defined at 3.3 kg (ICRP-89, ICRP110) and addresses the skin's epidermis, dermis, hair follicles, and glands.

Human skin shows higher variation in colour than any other single mammalian species and is the result of natural selection.

The red colour underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or the stimulation of the nervous system (anger, fear), arterioles dilate.

Areas that highlight higher amounts of UVR reflect darker-skinned populations, generally located nearer towards the equator.

For this reason it is thought that females may have evolved to have lighter skin in order to help their bodies absorb more calcium.

A validated comprehensive grading scale has categorized the clinical findings of skin ageing as laxity (sagging), rhytids (wrinkles), and the various facets of photoageing, including erythema (redness), and telangiectasia, dyspigmentation (brown discolouration), solar elastosis (yellowing), keratoses (abnormal growths) and poor texture.

In younger skin, sun damage will heal faster since the cells in the epidermis have a faster turnover rate, while in the older population the skin becomes thinner and the epidermis turnover rate for cell repair is lower, which may result in the dermis layer being damaged.

The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings.

[clarification needed] Certain diseases like shingles, caused by varicella-zoster infection, have pain sensations and eruptive rashes involving dermatomal distribution.

Potential medical applications of such particle transfer has prompted developments in nanomedicine and biology to increase skin permeability.

[46] Most particles that do penetrate will diffuse through skin cells, but some will travel down hair follicles and reach the dermis layer.

Ultraviolet radiation (UVR) slightly damages the surface of skin and causes a time-dependent defect allowing easier penetration of nanoparticles.

[51] The UVR's high energy causes a restructuring of cells, weakening the boundary between the stratum corneum and the epidermal layer.

[53] A large area of interest in nanomedicine is the transdermal patch because of the possibility of a painless application of therapeutic agents with very few side effects.

Development of techniques that increase skin permeability has led to more drugs that can be applied via transdermal patches and more options for patients.

Skin with high permeability allowed quantum dots with an antibody attached to the surface for active targeting to successfully penetrate and identify cancerous tumours in mice.

Tumour targeting is beneficial because the particles can be excited using fluorescence microscopy and emit light energy and heat that will destroy cancer cells.

[55] Sunblock—Sunblock is opaque and stronger than sunscreen, since it is able to block most of the UVA/UVB rays and radiation from the sun, and does not need to be reapplied several times in a day.

Vitamin A, also known as retinoids, benefits the skin by normalizing keratinization, downregulating sebum production, which contributes to acne, and reversing and treating photodamage, striae, and cellulite.

[57] Several scientific studies confirmed that changes in baseline nutritional status affects skin condition.

[58] Mayo Clinic lists foods they state help the skin: fruits and vegetables, whole-grains, dark leafy greens, nuts, and seeds.